The recent discovery that adult hematopoietic (blood) stem cells (HSCs) do not produce all immune cells despite sustaining blood production across the lifespan has fundamentally challenged our understanding of both immune development and function. Adoptive transfer studies and fate mapping experiments have demonstrated that tissue-resident macrophages are only specified during fetal development and cannot be generated or regenerated in adulthood. As compared to their adult HSC-derived counterparts, fetal-derived tissue resident macrophages reside and self-renew within their resident tissues, where they perform distinct homeostatic and immune surveillance functions. The discovery of a fetal origin of tissue-resident macrophages has raised many new questions about their specification, persistence across the lifespan, and distinct function. The long-term objective of this proposal is to define the developmental mechanisms underlying the unique functional capacity of tissue resident macrophages across the lifespan in order to illuminate their distinct role in both adult tissue homeostasis and disease. Recent work in our lab has identified interleukin-7 receptor (IL-7R) signaling as a novel mechanism regulating the establishment of tissue resident macrophages during fetal development. IL-7R signaling is restricted to regulation of the lymphoid lineage in adult hematopoiesis. The reliance of fetal myeloid specification on IL-7R signaling suggests that fetal hematopoiesis exploits alternate differentiation pathways in order to confer distinct characteristics on fetal-derived macrophages. In this proposal, we will capitalize on the discovery of this new regulatory mechanism to dig deeper into how these distinct immune cells are specified from fetal precursors during development. In Aim 1, we will use genetic approaches to determine the requirement for IL7R signaling during fetal myeloid specification. We will specifically delete the receptor sub-chain IL7raain a range of progenitors and mature cells at different stages of development to define how IL7r? regulates myeloid specification during fetal development. In Aim 2, we will define for the first time how IL-7R signaling occurs in myeloid cells - including downstream signal transducers and transcriptional targets- and the specific developmental processes that are regulated by IL-7R signaling during fetal macrophage establishment. In Aim 3, we will establish the function of IL-7 as a fetal myeloid niche factor and define both the requirement for IL-7 and which cell subsets are responsible for producing IL-7 within resident tissues to support macrophage development. Together, work from the proposed studies will illuminate how alternative differentiation pathways during fetal hematopoiesis impart distinct functional characteristics on fetal-derived tissue resident macrophages, and thereby illuminate the role of specific ontogenetic subsets of macrophages in normal tissue and disease processes.